This invention relates to a method of manufacturing composite materials and in particular to a method of manufacturing composite materials comprising reinforcing filaments enclosed in a matrix of a thermoplastic polymer.
It has long been common practice to manufacture composite materials which comprise reinforcing filaments enclosed in a matrix of a thermosetting polymer. Thus one particularly useful composite material comprises reinforcing filaments of carbon enclosed in a matrix of an epoxy resin. Whilst such composite materials have been extremely useful in replacing certain metals as constructional materials in the engineering industry, they do nevertheless suffer from certain disadvantages which makes their manufacture troublesome. For instance there are storage problems associated with thermosetting polymers prior to their incorporation into a matrix since they tend to have finite shelf lives and sometimes require to be stored in a refrigerated environment. Additionally, after a thermosetting polymer has been heated to an appropriate temperature, it hardens permanently, thereby effectively precluding its further shaping by deformation.
Thermoplastic polymers do not have storage problems since they have high molecular weights and are additionally thermoformable so that they can be re-worked many times after their initial moulding at temperature. Thus thermoplastic polymers have certain attractions as a replacement for thermosetting polymers as a matrix material in filament reinforced composite materials. However, there are difficulties in producing a composite material which comprises reinforcing filaments enclosed in a matrix of a thermoplastic polymer. One route for the manufacture of such composite materials entails impregnating the reinforcing filaments with a solution of the thermoplastic polymer in a suitable solvent. The solvent is then evaporated off and moulding carried out to provide the resultant composite material. Composite materials manufactured by such a method suffer from several disadvantages. For instance, it is very difficult to completely evaporate off the polymer solvent. Some trapping of the solvent may occur with the result that the composite material is to a certain extent, porous. This reduces its strength and sometimes leads to the accelerated thermal degradation of the polymer if the composite material is subjected in use to elevated temperatures. Another disadvantage is that it is not usually possible to accurately monitor the actual amount of polymer which is impregnated into the filaments. Some resin usually runs off the filaments with the result that the final polymer content of the composite material is lower than anticipated.
In U.K. Pat. No. 1,485,586 there is described a method of manufacturing a composite material in which filaments are interleaved between films of a thermoplastic polymer and the resultant sandwich structure subjected to heat and pressure so that the polymer flows and impregnates the filaments. This method has the advantage of not requiring the use of a solvent and consequently avoids the pitfalls associated with solvent use. However, if the polymer is of high viscosity at the temperatures usually employed in this type of method, then difficulty is encountered in achieving a satisfactory level of impregnation of the filaments by the polymer. Some separation of the fibres tends to occur so that the resultant composite material has a low filament level per unit volume of composite material, thereby adversely affecting its mechanical properties.
U.K. Pat. No. 1,570,000 describes a somewhat similar method of manufacturing a composite material, differing in that the filaments are impregnated with a solution of a thermoplastic polymer in a suitable solvent and the solvent evaporated off prior to the impregnated filaments being interleaved between films of a second thermoplastic polymer. Then, as in the previous method the resultant sandwich structure is subjected to heat and pressure to cause the thermoplastic polymer films to flow after which the assembly is cooled under pressure to avoid distortion of the resultant composite material. Thus the first thermoplastic polymer serves to bind the filaments together, thereby reducing the tendency for the filaments to separate during heating under compression. Thus there is less likelihood of the resultant composite material having a lower than expected filament level per unit volume of composite material. The teachings of U.K. Pat. No. 1,570,000 are directed to the impregnation of the filaments with a thermoplastic polymer so that there is a polymer pick-up by the filaments which, after solvent evaporation, amounts to 16 to 40% by weight. Unfortunately this means that whilst the method permits the use of films of high viscosity thermoplastic polymers, the resultant composite material is prone to solvent attack, even if the thermoplastic polymer of the film is of low solvent solubility. Thus the thermoplastic polymer originally applied in solvent solution is prone to solvent attack, thereby leading to increased likelihood of stress-cracking occurring in the resultant composite material. A further disadvantage of this method is that whilst the thermoplastic polymer used for the films may have desirable properties as a matrix material, the incorporation of a relatively large amount of a different thermoplastic polymer in the matrix may have a deleterious effect upon the properties of the resultant composite material.
It is an object of the present invention to provide a method of manufacturing a composite material wherein the aforementioned difficulties in manufacturing filament reinforced, thermoplastic polymer matrix composite materials are substantially avoided.